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We know that ETFs (like QQQ) track indexes (like $NDX). We know that on any given day, the ETF won't be exactly the price of the index. We know that over time, however, the ETF price will fluctuate around the index.
What I don't understand is why this ratio ($NDX:QQQ in this case) doesn't vary around some constant ratio. Instead, it shows an obvious negative slope over time. Other ratios ($SPX:SPY, $INDU:DIA) exhibit the same non-horizontal shape. One example is shown below.
I just want to know why the chart doesn't show a horizontal shape. (Is it related to the cumulative tracking error, and if so, how?)